1. Field of the Invention
The present invention generally relates to biometrics.
2. Related Art
A variety of well known techniques exist for sensing, measuring, and identifying biometric characteristics. These techniques focus on unique characteristics associated with structures that form the biometrics. By way of example, fingerprints, defined by ridges and valleys in a finger, are one such biometric.
As known to those of skill in the art, fingerprints are defined by unique structures on the surface of the finger called ridges and valleys. These ridges and valleys can be sensed, measured, and identified based upon a number of different modalities.
For example, some fingerprint measurement modalities rely on density values associated with the ridges and valleys. Others rely on dielectric permittivity as measured when an electric current if passed through the ridges and valleys. With respect to dielectric permittivity for example, the permittivity of a ridge (i.e., fingerprint tissue), is different from permittivity of a valley (i.e., air between the ridges).
Capacitive sensing is one technique that can be used to detect changes in permittivity. With capacitive sensing, capacitance values generated when a sensor plate (electrode) touches a ridge are different than those generated when the sensor is exposed to a valley.
Yet another modality is thermal conductance which is a measure of the temperature differences between the ridges and valleys. Optics are yet another modality. Optical techniques rely on an optical index of refractive and reflective changes between the ridges and the valleys.
Although the modalities differ, each approach seeks to accurately distinguish ridges from valleys in order to image the fingerprint. Some modalities, or techniques, are inherently more accurate that others relative to distinguishing ridges from valleys, as will be discussed more fully below. A relative assessment of this accuracy can be characterized in terms of contrast ratio. In a biometric context, contrast ratio is a measure of the contrast between tissue (i.e., fingerprint ridge) to air (i.e., fingerprint valley).
Viewed from another perspective, contrast ratio is an objective way to quantify potential differences in accuracy between thermal based modalities, those that rely on dielectric permittivity, from optical based modalities and others. The higher the contrast ratio, the greater potential for more accurate biometric sensing. When constructing a sensing system that incorporates, for example, one of the modalities noted above, designers must consider not only contrast ratio, but manufacturability, along with costs.
Thermal and dielectric permittivity based sensing systems, for example, have relatively low contrast ratios, as will be discussed more fully below. That is, under ideal conditions and with the utmost care and consideration during design and/or manufacturing, these systems are inherently limited in the accuracy of their measurement output data.
What is needed, therefore, are highly reliable techniques for sensing biometrics. What are also needed are techniques for sensing biometrics, such a fingerprint ridges and valleys that have higher contrast ratios than traditional sensing systems.